The invention relates to the internals for a radial flow reactor. A radial flow reactor includes devices for retaining catalyst or adsorbent in a space within a reactor, and devices for the inlet and outlet flow of fluid across the catalyst or adsorbent.
A wide variety of processes use radial flow reactors to provide for contact between a fluid and a solid. The solid usually comprises a catalytic material on which the fluid reacts to form a product, or an adsorbent for selectively removing a component from the fluid. The processes cover a range of processes, including hydrocarbon conversion, gas treatment, and adsorption for separation.
Radial flow reactors are constructed such that the reactor has an annular structure and that there are annular distribution and collection devices. The devices for distribution and collection incorporate some type of screened surface. The screened surface is for holding catalyst or adsorbent beds in place and for aiding in the distribution of pressure over the surface of the reactor, or adsorber, and to facilitate radial flow through the reactor bed. The screen can be a mesh, either wire or other material, or a punched plate. For a moving bed, the screen or mesh provides a barrier to prevent the loss of solid catalyst particles while allowing fluid to flow through the bed. The screen requires that the holes for allowing fluid through are sufficiently small to prevent the solid from flowing across the screen. Solid catalyst particles are added at the top, and flow through the apparatus and removed at the bottom, while passing through a screened-in enclosure that permits the flow of fluid over the catalyst. The screen is preferably constructed of a non-reactive material, but in reality the screen often undergoes some reaction through corrosion, and over time problems arise from the corroded screen or mesh.
The screens or meshes used to hold the catalyst particles within a bed are sized to have apertures sufficiently small that the particles cannot pass through. The outer screen element can be provided by a cylindrical screen that retains particles in its interior, and provides for the distribution of fluid through the space between the screen and the outer wall of the reactor. Another design for an outer screen element is to use a plurality of oblong conduits arrayed around the wall of the reactor. A common shape for the oblong conduits is a scallop shaped cross-section where the flattened side is positioned against the wall of the reactor and the more sharply curved side presents a screened face that allows the catalyst to flow against, while fluid flows within the oblong conduit and passes through the screened face. The flattened side is shaped to substantially conform to the curve of the reactor wall to minimize volume between the conduits and the reactor wall.
The common type scallop design can be found in U.S. Pat. Nos. 5,366,704 and 6,224,838, where the scallops have either punched plates or longitudinally extended profile wire arrangements, respectively.
A significant fabrication and maintenance concern for scallop designs is the accommodation of differential thermal growth between the reactor shell and the scallop internals. This is accomplished in conventional scallops using a machined riser with a machined seal plate that has a gap allowing for independent movement of the two components.
FIG. 1 illustrates one embodiment of a radial flow reactor 100 with a typical design for the scallop 105. The outer conduit 110 has a back wall 115 which is adjacent to the vessel wall 120. At least a portion of the front side 125 of the outer conduit 110 comprises a screen section 130 (e.g., a wire arrangement or perforated plate, or the like) and a solid section 135. The outer conduit 110 rests on the outer conduit bottom support 137.
There is a central conduit 140 which has a front side 145, including a screen section 150 and a solid section 155.
Between the front side 125 of the outer conduit 110 and the front side 145 of the central conduit 140 is the catalyst bed 160. The shape of the catalyst bed 160 depends on the shape of the outer conduit 110 and central conduit 140.
The riser tube 165 is attached to the outer conduit 110, typically by welding forming a single piece.
The inlet vapor flows through a riser tube 165 into outer conduit 110, through the catalyst bed 160, and out through the central conduit 140.
There is a scallop support ring 170 with a seal plate 175 on it. The riser tube 165 is positioned in an opening in the seal plate 175. The seal plate gap 180 between the seal plate 175 and the riser tube 165 is set quite narrow, e.g., about 1 mm (which is smaller than a catalyst pill) to ensure near isolation of the regions for both hydraulic and catalyst containment. There is a much larger support ring gap 185, e.g., about 13 mm, between the scallop support ring 170 and the riser tube 165. The support ring gap 185 is set by the installation guidelines for the scallops.
This design also includes a small seal gas flow (e.g., about 1% to about 5% of the incoming gas) into a seal vent basket 190 attached to cover plates 191. The purpose of the seal gas is to ensure there is a downward vapor flow through the seal catalyst 195 to prevent uplift of the catalyst from the inlet vapor flow flowing through the seal catalyst at 195 the top of the catalyst bed 160. The seal vent basket 190 is carefully sized to control the 1%-5% flow into it.
In FIG. 1, pressure P1 is upstream of the riser tube 165, and pressure P2 is between the scallop support ring 170 and the top of the seal catalyst 195. P1 is larger than P2, and the difference between P1 and P2 can be about 7 kPa (1 psi). The reason for this pressure differential is the pressure drop through the riser tube 165. This pressure drop is the driving force for the vapor bypass in FIG. 1. The larger the pressure drop is, the greater the driving force for vapor bypass will be.
If the seal plate 175 fails, and the seal plate gap 180 increases above 1 mm, two problems may occur. One is that some of the inlet vapor flow may bypass the riser tube 165 and jet down onto the top of the seal catalyst 195. The increased vapor flow can cause fluidization of the seal catalyst 195 and can lead to plugging of the screen section 130 of the outer conduit 110 and/or the screen section 150 of the central conduit 140 which may require shutting the unit down to remove the catalyst fines.
The second consequence is that fluidized catalyst may enter the seal plate gap 180 and then either plug the seal vent basket 190 and/or fill up the outer conduit 110, either of which will negatively affect the reactor performance and potentially cause a unit shutdown.
Another problem with this design is that the scallop 105 and seal plate 175 can be difficult to install. The riser tube 165 and the outer conduit 110 have to be aligned with a hole in the scallop support ring 170 having a clearance of about 13 mm. Then the seal plate 175, which has about a 1 mm clearance, must be installed and tack welded in place. This installation process can be difficult because the outer conduit 110 can weigh up to 227 kg (500 lb.) and be 15.2 m (50 ft.) long. In addition, the outer conduits 110 are sometimes slightly warped, and the vessel can be out of round. As a result, there is little margin for error in the installation for this design.
Therefore, there is a need for an improved design for the outer conduit and radial flow reactor using them.